One well known method of air separation is by pressure swing adsorption (PSA). One known PSA process for separation of oxygen from air employs a zeolite molecular sieve adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its preferential adsorbtion of nitrogen. In operation, a bed of this adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed, some of the nitrogen and a smaller proportion of the oxygen and argon plus substantially all the carbon dioxide and water vapour in the feedstock are adsorbed, and an oxygen-rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and then evacuated through its inlet, so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step. Typically, but not necessarily, two or more adsorbent beds are employed and operated on similar cycles but sequenced to be out of phase with one another, so that when one bed is on its adsorption step an other bed is on its desorption step, and vice versa.
It is typically found that although a good separation can be obtained as between oxygen and nitrogen, the oxygen product is enriched in argon as compared with the original air. Indeed, in some typical PSA plants for separating oxygen from air the resulting oxygen may contain in the order of 4 to 5% by volume of argon. Occasionally, there arises a use for the oxygen in which such a level of argon is either in fact disadvantageous or perceived by some users as being disadvantageous. There is for example a demand in the field of anaesthesia for a PSA method of producing oxygen whereby a resulting product containing less than 4% by volume of argon is produced. One reason for the demand is that the closer the oxygen purity is to 100% the more closely the oxygen flow rate reading given by the oxygen flow meter on standard anaesthetic apparatus will correspond to the actual flow of molecular oxygen. It is an aim of the present invention to meet this need.